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Garage ceiling insulation

12K views 16 replies 5 participants last post by  Windows on Wash 
#1 ·
Hi all, first time poster! I did a few searches, but haven't found an answer to what I'm looking for, so hope y'all can be of assistance.

We have an older detached garage built in the 60's that we're wanting to convert into a gym/workshop space to use year-round. I've started off by insulating the walls and putting up drywall, but am at a bit of a loss as to what to do with the ceiling.

I want to keep the cathedral-style ceiling because I'm fairly tall and wouldn't be able to run on a treadmill or jump if we just closed off the rood - but the joists that run up the roof are 2x4's - meaning I can only fit R-13 faced insulation between them? I'm not 100% sure. I'd love to fit thicker stuff between them, but I know I'm not supposed to compress the insulation.

Anyone have any ideas on how to get the best bang for my buck insulation-wise up there? I was also planning on cutting some soffit vents into the side and running channels up behind the insulation to get some airflow in there - but with only 3 1/2"-4" to work with, that really limits the space I can use. Ideas?

Thanks all! Please direct me to any existing threads if there any on this, in case I missed them too!
 
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#3 ·
Thanks for the reply! I realize that R-13 is too little, and I'm wondering what else I can do with the space I have? I really do want to keep it cathedral-style (I hope I'm using the right term there) because if we close off the ceiling, I'm pretty tall and won't be able to use the space as well as I'd like to.

If I use thicker insulation, and it sticks out from between the joists but is held in place by a plastic sheet that would be used as a vapor barrier - would that work? Does the insulation have to fit in between the joists?
 
#5 ·
The right way to do it would be add collar ties to act as a ceiling at a height you can live with, add 2 X 2's to the bottoms of the rafters, Add foam baffles from where the ridge vents will be to the point where the collar ties are.
Now you can add the R13 and not compress it and still have air flow. On the flat part of the ceiling you could go as thick as you want to.
You also going to need to add a ridge vent so the hot air can escape in the summer.
If you install insulation tight againt the sheathing it will super heat the shingles and shorten there life by about 5 years and the heat in the summer will transfure down into the room.
By having that flat raised ceiling you could add can lights or a ceiling fan.
 
#6 ·
I would agree with about 95% of this with the exception that there are very slight differences in shingle temps and no evidence to indicate that unvented assemblies shorten shingle life.

Unvented assemblies that leak moisture with shorten roof life in general but there is nothing to indicate that shingle life suffers.
 
#7 · (Edited)
90 % of my work is remodeling in older houses 100 years old or older. Dozens of times I've seen where over the year DIYS have gone in there and tryed packing in the insulation to tight with no soffit vents, no baffles and no ridge vent, The paper on the insulation got so dry out and had gotten so hot it was falling apart, the 10 year old architural shingles would crumble in you hand and the granuals just filled the gutters.
http://www.gaf.com/Roofing/Residential/Key-Danger-Signals/key-danger-signals.aspx
Notice how they show at least 4, differant pictures of what poor venting can do.
 
#8 ·
Poor ventilation in a vented assembly and a hot/insulated roof deck are two totally different things.

Shingle manufacturers will forever attempt to blame the conditions and not the shingle.

There is no data to indicate that an insulated, sealed roof assembly shortens shingle life.

Most shingle cooling happens to the exterior and there are studies showing only a 5-7° difference in peak shingle temperature in Las Vegas.

Ventilation and moisture drive to the back of the shingle can kill it early.
 
#10 ·
No shingle manufacturer, roof inspector, or qualified roofer would ever tell you that it is ok to insulate against the deck without ventilation. Heat causes breakdown of anything in any application, so why are shingles any different?

This is a literature review by the Florida Solar Energy Centre. It does not cover shingle life in much detail, but it does compare traditional vented, unvented, and cathedral vented roofing systems. They all have their strong points but the cathedral vented is the recommended.
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
 
#11 ·
No shingle manufacturer, roof inspector, or qualified roofer would ever tell you that it is ok to insulate against the deck without ventilation. Heat causes breakdown of anything in any application, so why are shingles any different?

This is a literature review by the Florida Solar Energy Centre. It does not cover shingle life in much detail, but it does compare traditional vented, unvented, and cathedral vented roofing systems. They all have their strong points but the cathedral vented is the recommended.
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf
Shazapple,

All of the science and data that is being collected today is in complete contradiction to your assertions.

The idea of ventilated assemblies in Fl and other hot and humid climates was based on very antiquated recommendations and using the shingle manufacturers of build code inspectors as you source of information is flawed at its source.

95% of the shingle cooling happens to the exterior surface via convection.

I have never seen a report, although cited, showing a 32-40° difference between and unvented and vented assembly. As a matter of fact, orientation, color, and material have everything to do with surface temperatures. That being said, Las Vegas shingle roofs that were either well ventilated (1:150) vs. completely sealed only show a 9° difference in surface temperature (well withing guidelines and allowable maximums).

Lets be clear about one thing, attic ventilation is for moisture. It always has been and will continue to do so. Removing moisture to prevent rot, mildew and mold is all it is intended for.

Beside the energy efficiency reductions in a vented assembly (i.e. wind stripping of R-value, more building leakage, ice damns in low slope assemblies), vented assemblies can also entry pathway points for rain and bulk moisture and embers from brush fires.

Don't get me wrong, if you are going to run a sealed attic/hot roof the details need to be correct. The vapor diffusion concerns are more pressing in colder climates.

http://www.thermalsealexperts.com/air03.php

http://www.buildingscience.com/docu...as-nevada/view?topic=/doctypes/researchreport

If surface temperature really ruined asphalt...wouldn't the roads be crumbling much faster?
 
#16 · (Edited)
By the way...if anyone can show me an 30-year shingle that has made it that long...you will be the first.

You will forgive me if I take every shingle manufacturer's warranty claims and exclusions with a grain of salt.

Interesting that the most significant piece of evidence for shingle degradation was...

Perhaps the most rigorous analysis of the impact of attic ventilation on shingle life comes from work
sponsored by the Certainteed Corporation (Shiao et al, 2003) in which the authors developed a mathematical model of cumulative shingle damage to evaluate the kinetics of roofing material degradation. Again, this work showed that fundamental increases to molecular activity were the fundamental driving force in reducing shingle life expectancy. In particularly, the high temperature history in a hot climate was showed accelerated aging of composition shingles. For instance, the higher temperatures in a hot climate such as Miami, were shown to accelerate aging during during winter months at a rate about 10 times faster than in Minneapolis. The effect of attic ventilation was found to reduce the uneven distribution of cumulative damage across the roof deck due to unbalanced thermal regimes in the unvented roof deck as well as to reduce the times at higher temperatures. The work did not look at how no ventilation and an insulated roof deck would influence the time-temperature history, but there is little doubt that such an evaluation would find even larger differences in the rate of cumulative shingle aging (Shiao, 2005).


Again, the point being that region has much more impact that ventilated vs. sealed.
 
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